Conventional conveyor belts offer an economical method for transporting bulk material at certain included angles ranging from a low of 7.degree. for fine and somewhat lubricious material such as soda ash brickets, to a high of 30.degree. for cinder concrete and ground phosphate fertilizers. Typical recommended inclination angles for open-pit mine products such as excavated earth including anthracite coal, bituminous coal, lignite and crushed rock, vary from 15.degree. to 22.degree. with respective angles of repose from 29.degree. to 44.degree..
The conventional conveyor is often the most economical, reliable and safe means for transporting such bulk materials. There are, however, many cases which strongly warrant an increase in conveying angles.
In a static case, a cohesionless material on a rubber conveyor belt will begin to slide back when the incline angle of the belt surface just exceeds the angle of internal friction of the material or the friction angle of the material to belt surface interface, whichever is smaller. The angle of internal friction is equal to the angle of repose of such materials.
Both the angle of repose and the friction angle for bulk materials on rubber will vary from one material to the next and will be affected, even for the same type of material, by the maximum lump size, the lump size distribution, the orientation of the conveyor cross-section, and the shape that the particles or lumps take as a result of the reduction process, i.e., blasting and the varying degrees and methods of crushing.
The recommended conveying angles are in general far below the recommended friction angles. This is due to the dynamics induced in a moving belt conveyor, which result in relative motion between adjacent particles or lumps of the bulk material and between the material and carrying surface of the conveyor belt.
Various prior art approaches have been disclosed in an effort to convey particular material at high angles. All of these prior art disclosures are directed to solving the problem of conveying bulk material at steep angles and high conveying rates efficiently and economically. None, however, have received commercial acceptance for one reason or another.
In designing such a system and improving the prior art devices to a structure of commercial acceptability the conveyor geometry must lend itself to support and extend along a straight incline. The conveyor surfaces must lend themselves to continuous cleaning and easy repair.
The prior art devices include bucket-elevators, flat belts with corrugated rubber sidewalls and cross-cleats, and troughed belts with fins, all of which may be used to convey bulk materials along steep inclines. Presently these methods are typically expensive, of limited capacity, and are unable to completely discharge sticky material. Further, they do not lend themselves to continuous cleaning by belt scrapers or plows because of the cleats, fins, and the like.
The problems as stated above have been solved, at least in theory, in the past by the sandwich belt conveyors consisting of a troughed conveyor belt working in association with a cover or hugging belt. Hugging pressure is required to develop sufficient friction between bulk material and belt surface so that the bulk material will not slide back when conveyed at steep angles.
Various methods have been used or suggested to provide the required hugging pressure. These include the use of a heavy cover belt whose normal component of its linear weight provides the hugging force necessary. This method is disclosed in U.S. Pat. No. 3,618,748 to Suloff.
This method provides uneconomical solutions on conveying angles approaching 45.degree.. The normal component of the belt weight decreases with increasing conveying angle but the hugging pressure requirements increase. Very heavy belts are therefore required and such belts are expensive to manufacture. The support structure also becomes expensive because of the additional loading by the heavy cover belt.
Another method for providing the required hugging pressure is in the use of pressing rolls which, when pressed onto an ordinary cover belt, provides the hugging force necessary. This method is disclosed in USSR Pat. No. 502,803 to Usov.
This method describes a concept of the past which included two and four individually pressed rubber tires at each cross-section of the conveyor with wide spacing along the conveyor length. Pressing concepts of the past did not distribute the load sufficiently over the conveyor belt surface and this resulted in load concentrations, localized wear and accelerated breakdowns of conveyor components as well as in the loss of hugging pressure between the widely spaced rolls along the conveyor length. This resulted in material spillage. Cover belt tension could not be increased to offset these problems because of the nature of the conveyor loading area.
Yet another method for providing hugging pressure is in the use of two belts which resist lateral bending and are pressed together at the edges by staggering edge rolls. In this manner the lateral flexural stiffness of the belt must be such that when the conveyed material enters the sandwich and pries them apart in the middle region the resistance to prying provides the hugging pressure necessary to prevent the material from sliding back under the forces of gravity as permitted by overcoming the frictional coefficient. This method is described in U.S. Pat. No. 3,982,626 to Mehta and German Pat. No. 1,259,238 to Pelzer.
This third method is for conveying materials vertically only. Further, it has capacity limitations. At capacities which are high, very wide belts are required. The lateral bending stiffness to provide the required prying resistance in this hugging pressure must be very high. Because such belts must be very thick with many plies of lateral reinforcements, such belts become very expensive to manufacture. In addition, such stiff belts will not sit in a troughed configuration when, at the loading regions, it must behave as an ordinary troughed conveyor. Practical belt widths are limited to approximately 36" and conveying capacity is less than a thousand tons per hour even when conveying very dense material at relatively high speeds.
The last method for providing the required hugging pressure is in the use of two ordinary belts at low-elastic modulus and the judicious selection of a conveyor profile geometry so that one belt is supported by troughing idlers in a convex vertical curve. The other belt provides the hugging pressure necessary by the radial pressure and by virtue of its belt tension and the profile geometry when formed in a C-shaped or snake-like configuration. This method is described in U.S. Pat. Nos. 2,642,178 and 3,805,946 to Naylor and Yateman, respectively, and the Report "Evolution of Sandwich Belt High Angle Conveyors" by Joseph A. Dos Santos and Earl M. Frizzell.
With this method the required hugging pressure is derived by the judicious selection of a conveying profile geometry which will exploit the inherent belt tension to produce a radial hugging pressure which will prevent sliding of the material in the sandwich when conveying at high angles. The nature of the loading area permits the selection of the belt tension in the region which is consistent with the selected conveyor profile and the required radial pressure. The C-shaped loop belt elevator as described in U.S. Patent to Naylor and Yateman, referred to above, includes structures embodying this method for concept. This has no practical capacity limitations. The C-shaped geometry, however, is impractical for conveying material along a straight line path of predetermined incline.
An extension of the concept referred to in the immediately preceeding paragraph is the snake sandwich conveyor described in the aforementioned Report. This development has solved some of the problems of geometrical conformance by introducing an inflection to the conveyor belt. By inflection it is meant curvature reversal points along the conveyor profile and alternately carrying the top and bottom belts on troughing idlers along a vertical convex curve, with the belt providing the radial pressure by virtue of its belt tension.
The snake sandwich conveyor concept permits conformance to any vertical conveying geometry by the introduction of the inflexion points as required along the length. A correctly conforming geometry with loading and discharging at pre-specified locations is determined by trial and error. Once such a conveyor is installed, the profile geometry cannot be altered appreciably and such a conveyor cannot be extended or shortened. Such conveyors are impractical for applications requiring a high degree of mobility and flexibility.